The present invention relates to an optical transmission substrate, a method for manufacturing an optical transmission substrate and an optoelectronic integrated circuit. Particularly, the present invention relates to an optical transmission substrate including an optical waveguide therein, a method for manufacturing an optical transmission substrate and an optoelectronic integrated circuit.
The technological innovations over the years are reaching a territory where computer performance is subjected to rate-controlling depending on how fast data transfer to/from other elements is performed and not on improvement in a switching speed of a semiconductor element or an operating clock. Thus, connection within a board or between boards by use of a terabit-class ultra-high speed, large-capacity interconnect has been required. Along with the above requirement, it has been more and more difficult to realize sufficient performance of even interconnect within the board by use of electric interconnect.
On the other hand, optical interconnect has a characteristic that the optical interconnect can perform ultra-high speed transmission exceeding 10 Gb/s by direct modulation of a laser and has a small space occupied volume per channel. Thus, it has been expected that interconnect in a short transmission distance of about several 10 cm may also have a sufficient advantage over the electric interconnect. However, a high-performance mounting board, in which the electric interconnect and the optical interconnect are mixed by use of a possible method as an industrialization process, has not yet been realized. Thus, urgent development has been expected.
As a method for providing optical interconnect on a mounting substrate, the following technologies have been disclosed.
(1) Method by Installing Optical Fibers
A method for installing optical fibers on a mounting substrate by using an electric wiring installation machine and a one-stroke drawing technique is disclosed (patent documents 1 and 2).
(2) Method by using Optical Waveguide
An optical waveguide can be formed by high-precision processing utilizing a semiconductor process such as spin coating and photolithography. Thus, the optical waveguide may be realized at low cost. Consequently, research and development of optical waveguides by use of various materials have been conducted.
A quartz optical waveguide is excellent in heat resistance with few loss of light absorption and has been put to practical use as an optical demultiplexer, an add/drop selector or the like in a backbone optical communication field using single-mode transmission (nonpatent document 1).
An optical waveguide by use of an organic polymer material can be formed by spin coating a film of the organic polymer material with a sufficient thickness. Moreover, photolithography can be utilized for pattern formation. Thus, there is an advantage that mounting substrates can be mass-produced at a low price.
Patent document 3 discloses a method for connecting an optical waveguide with an optical element in the case of using the optical waveguide in optical interconnect. According to this document, in order to allow light guided by an optical waveguide provided parallel to a substrate to enter into an optical element by bending the light in a vertical direction of the substrate, a surface tilted at 45 degrees to the optical waveguide is formed by cutting a core part of the optical waveguide at an end of the optical waveguide. Accordingly, after applying metal on this surface to obtain a reflection surface, clad is formed in a portion in the vicinity of the reflection surface, the portion being cut to form the reflection surface, and above the core part of the optical waveguide. Subsequently, a light receiving element is provided on an upper surface of the clad.
Patent document 4 discloses a method for connecting an optical element and an optical waveguide with each other in such a manner that the optical element is provided in an upper surface of a mounting substrate, the optical waveguide is provided in a lower surface of the mounting substrate and a through-hole is made to have an optical waveguide function, in order to eliminate instability in mounting the optical element directly on the optical waveguide. Besides the above-described method, a method using a structure in which an optical waveguide is exposed to one of an edge of a mounting substrate is disclosed in patent documents 5 and 6.
When the optical waveguide is provided on the surface of the mounting substrate, the optical waveguide is damaged in mounting electronic components or optical components and a difference in thermal expansion coefficient between the mounting substrate and an optical waveguide material causes the mounting substrate to bend. Accordingly, the optical waveguide is damaged and an optical coupling efficiency between the optical components and the optical waveguide is lowered. Thus, a method for resolving the problem described above by burying the optical waveguide in the mounting substrate has been studied.
Nonpatent document 1 discloses a method for connecting an optical waveguide and an electronic circuit with each other by use of an optical guide pin, which is forming the optical waveguide bent at 90 degrees. More specifically, a round through-hole is made in the optical waveguide and an optical guide pin formed of a step-index multimode optical fiber with a core diameter of 50 micrometers, which is previously polished at an angle of 45 degrees, is inserted into the through-hole. Here, the optical waveguide adopts a structure in which a periphery of a core of a 40×40 μm square is covered with clad.
Nonpatent document 2 discloses a method for optically coupling signal light to a surface-mounted optical device in such a manner that light is bent at 90 degrees by use of a 45-degree mirror formed at an end of an optical waveguide sandwiched between printed circuit boards, the signal light is extracted in a vertical direction from the printed circuit boards and the signal light is converged by a lens.
Nonpatent document 3 discloses an optical transmitter receiver module mounting a polymer optical waveguide on an electronic circuit.
(Patent Document 1)Japanese Patent Laid-Open No. Hei 11 (1999)-119033
(Patent Document 2)Japanese Patent Laid-Open No. Hei 11 (1999)-119034
(Patent Document 3)Japanese Patent Laid-Open No. 2000-47044
(Patent Document 4)Japanese Patent Laid-Open No. 2000-81524
(Patent Document 5)Japanese Patent Laid-Open No. 2000-227524
(Patent Document 6)Japanese Patent Laid-Open No. 2000-235127
(Nonpatent Document 1)B. J. Offrein et. al., “Tunable WDM Add/Drop Components in Silicon Oxynitride Waveguide Technology”, 49th Electronic Components & Technology Conference 1999 Proceedings, p. 19-25
(Nonpatent Document 2)Mikami, Uchida, “Development in Optical Surface-Mount Technology”, IEICE (Institute of Electronics, Information and Communication Engineers) Transactions C Vol. J84-C, p. 715-726, 2001
(Nonpatent Document 3)Ishii, Arai, “Wide Tolerance ‘Optical Bump’ Interface for Chip-Level Optical Interconnection”, IEICE (Institute of Electronics, Information and Communication Engineers) Transactions C Vol. J84-C, p. 793-799, 2001
(Nonpatent Document 4)Maruno, “Polymer Optical Waveguide Device”, IEICE (Institute of Electronics, Information and Communication Engineers) Transactions C Vol. J84-C, p. 1-6, 2001
(Nonpatent Document 5)R. F. Cregan et. al., “Single-Mode Photonic Band Gap Guidance of Light in Air”, Science, Vol. 285, p. 1537-1539, 1999
In the method of patent documents 1 and 2, it takes time to sequentially install the optical fibers and it is difficult to mechanize a method for forming an input/output portion of light to the optical fibers. Thus, the method is not suitable for mass production. In addition, the input/output portion of the optical fiber is mechanically weak and easily damaged. Moreover, it takes effort to exchange the optical fibers when damaged. Furthermore, it is impossible to perform wiring by use of, for example, a radius of curvature (for example, about 20 mm or less) which is smaller than the least curvature of the optical fiber. Consequently, application to a high-density optical/electric mounting board is difficult.
Moreover, in the method of nonpatent document 1, it is required to perform sputtering or the like for attaching films and etching an optical waveguide structure. In addition, large-sized vacuum equipment is required in the conventional manufacturing method. Thus, this method is not suitable as a method for forming an optical waveguide in a large mounting board. Moreover, there is a problem that it is difficult to thicken the optical waveguide structure by sputtering or the like in order to form a 50×50 μm square multimode optical waveguide which is easily subjected to optical coupling.
Moreover, in the method of patent document 3, the clad is interposed between the core part and the reflection surface and between the reflection surface and the light receiving element. Thus, light emitted from the optical waveguide is diffused and irradiated on the light receiving element. Moreover, light of a light emitting element is optically coupled without special focusing optics or guiding optics. Thus, it is difficult to couple light of the optical waveguide to the optical element through a thick mounting substrate. Moreover, the optical waveguide and the optical element are exposed to the surface of the mounting substrate. Thus, there is a problem that it is highly likely that the optical waveguide and the optical element are damaged by mechanical, thermal and chemical processing in a laminate process, a built-up process and the like for an electronic circuit, which should be performed after formation of the optical waveguide.
Furthermore, in the method of patent document 4, in the case of performing a process of forming the optical waveguide after mounting an electronic device, there is a possibility that the electronic devices are damaged. Meanwhile, in the case of mounting the electronic device after forming the optical waveguide, there is a possibility that the optical waveguide exposed to a lower surface of the mounting substrate is damaged. Moreover, in the methods of patent documents 4, 5 and 6, the optical waveguide portion and the mounting substrate have asymmetric structures. Thus, there is a possibility that the optical waveguide is damaged by warp in the mounting substrate caused by temperature rise in the middle of a manufacturing process of the mounting substrate, temperature rise within a case in operation of the electronic device and the like.
In the method of nonpatent document 2, a core of an optical waveguide and a core of an optical fiber are connected to each other through clad of the optical fiber. Thus, compared to the case of connecting the both cores directly to each other, light is diffused and coupling loss occurs. Moreover, the optical waveguide and a optical guide pin have different core diameters from each other. Thus, compared to the case of connecting a thinner one with a thicker one, in an opposite case, there is a possibility that the loss is increased by about 10% and the coupling efficiency is reduced to 50% or less. Moreover, it is required to reduce a diameter of an optical detector in order to perform high-speed optical transmission. Thus, the coupling loss is increased.
In addition, in making a through-hole, the end of the optical waveguide is processed. Thus, there is a possibility that an optical output portion is damaged by chips including chips of electrodes or the like. As a result, light scatters in the optical output portion and the coupling efficiency is further lowered.
Moreover, in order to provide both of the optical interconnect and the electric interconnect, the mounting substrate mounting the optical waveguide adopts a structure in which upper and lower sides of the optical waveguide are sandwiched by printed circuit boards. Thus, it is difficult to align a cross-section portion of a tip of an optical guide pin with a height of a core portion of the optical waveguide. Furthermore, the printed circuit board has a large thermal expansion coefficient in its thickness direction. Accordingly, a position of the optical guide pin is shifted by a temperature change. Thus, the coupling loss is increased.
As described above, in the method for aligning the cross-section portion of the tip of the optical guide pin with the height of the core portion of the optical waveguide by making the through-hole and inserting the optical guide pin into the through-hole thereabove, the alignment is difficult and the coupling loss occurs. Thus, it is difficult to realize a high-performance optical/electric mounting substrate.
Moreover, in the method of nonpatent document 3, it is difficult to realize a sufficient coupling efficiency in the case where light is focused in a thickness direction of a thick mounting substrate by use of a lens relay system and the light is received by a photodetector. Particularly, in the case of realizing ultra-high-speed transmission, it is required to reduce a diameter of the photodetector. Thus, light focusing by use of the lens relay system becomes extremely difficult.
Moreover, in the method of nonpatent document 4, in the formation of an optical/electric mounting substrate by mounting a polymer optical waveguide on an electronic circuit, the exposed optical waveguide is damaged in mounting electronic components. Moreover, deterioration and warp are caused by heat. Thus, it is difficult to realize a high-performance mounting substrate.